1. Field of the Invention
This invention relates to flow meters and more particularly to ultrasonic flow meters, which term is intended to cover fluid presence sensors, i.e. sensors which indicate the presence of a fluid as opposed to a continuous flow of a fluid.
2. Description of the Prior Art
Ultrasonic flow meters are well known and comprise ultrasonic transducer means mounted in relation to a conduit, through which a fluid flows, or in which a fluid is present from time to time, such that an ultrasonic signal can be transmitted across the conduit. Usually, a pair of ultrasonic transducers is employed, the two transducers being mounted on opposed sides of the conduit with one acting as an ultrasonic transmitter and the other as an ultrasonic receiver. Assuming a cylindrical conduit, the two transducers are mounted diametrically opposite and extreme care has been taken in the past to ensure that the ultrasonic signal emitted by the transmitter is directed across the conduit, and hence through any fluid therein, as opposed to around the wall of the conduit, the latter being the path of least resistance and thus the "preferred" path of the ultrasonic signals. To this end, each transducer has been mounted in a cup inserted into the wall of the conduit, the cup being sealded into that wall with a silicon-based sealant, for example. If the cups are metallic, then it is usual to provide a non-metallic mount therefor, again to safeguard against the risk of an acoustic short circuit around the wall of the conduit.
These precautions against acoustic short circuiting are tedious but more importantly, it is impossible to achieve a leak-proof seal between the cup and conduit for certain fluids and/or pressures. One of the most difficult fluids in this respect is steam. Thus whilst the short circuiting problem is solved by the use of an acoustic insulator between each transducer and the conduit by way of the non-metallic mount and/or sealant, the seal achieved is not viable under certain conditions.